Acoustic consumption monitor

Abstract

A method of determining the reaction efficiency of a CVD process reactor by which a reactive gas combination entering the reactor deposits a solid reaction product as a thin film onto a substrate. The composition of the reactive gas combination is measured by an acoustic cell which is disposed downstream from the outlet port of the reactor to determine the reactor's efficiency. Determination of the reactor's efficiency allows the growth rate of the thin film to be determined as well as associated rates of the products which have been exhausted from the reactor.

We claim:  
     
       1. A method of calculating the composition of gases exhausted from a CVD process reactor, said method comprising the steps of: 
       measuring the inlet composition of a gas composition entering the reactor using a first acoustic cell disposed in relation to an inlet side of said reactor;  
       measuring the composition of gas mixture exiting said reactor using a second acoustic cell disposed in relation to an outlet side of said reactor, said second acoustic cell being disposed downstream of at least one vacuum pump disposed between the outlet side of said reactor and said second acoustic cell;  
       determining the reaction efficiency of the reactor from the measured inlet side and outlet side gas compositions;  
       determining the unreacted mass fraction of the exhausted gas components;  
       determining the composition of the reacted gas components; and  
       adding the reacted and unreacted gas components to determine the total exhausted gases.  
     
     
       2. A method as recited in  claim 1 , wherein the unreacted fraction is calculated from the following relationship: 
       
         
             W (1− E)    
         
       
       in which: 
       W=input mass flow rate of precursor; and  
       E=reaction efficiency for the chemical reaction aA+aB⇄cC+dD.  
     
     
       3. An apparatus for determining the reaction efficiency of a CVD reactor, said reactor having a defined cavity for retaining at least one substrate onto which is deposited a solid reaction product from a reacted binary gas mixture (aA+bB⇄cC+dD), said reactor further including an inlet side and an outlet side, said apparatus comprising: 
       a first acoustic cell for determining the composition of said gas mixture at the inlet side of the reactor;  
       a second acoustic cell for determining the composition of said gas mixture at the inlet side of the reactor;  
       pump means disposed between the outlet side of said reactor and said second acoustic cell; and  
       a bypass line extending directly between the first and second acoustic cells;  
       each said acoustic cell being capable of measuring relative differences in the speed of sound for gases passing therethrough, in which the reacted product is derived from the relation:            (         c   _     o       c   _       )     2     =             (     1   -   Δ     )          M   A       +       (     x   -       b   a        Δ       )          M   B       +     c        Δ   a          M   C             M   A     +     x                   M   B           ·         [       1   +   x       1   +   x   +       Δ   a          (     c   -   b   -   a     )           ]     2          [           1   -   Δ       ϒ   A       +       x   -       b   a        Δ         ϒ   B       +     c        Δ     ϒ   C             (       1   /     ϒ   A       +     x   /     ϒ   B         )       ]                         
       in which: 
       Δ=reacted product;  
       M A , M B , M C =molecular weights of A, B, C;  
       γ A , γ B , γ C =specific heat ratios of A, B, C;  
       x=mole fraction of A; and        (         c   _     o       c   _       )                   
        is the frequency ratio of the gas mixture between a reacted mode and a bypass mode.  
     
     
       4. Apparatus as recited in  claim 3 , including at least one mass flow meter for controlling the mole fraction of A entering said inlet side. 
     
     
       5. Apparatus as recited in  claim 3 , wherein said pump means includes a turbomolecular pump and a diaphragm pump disposed between the outlet of said reactor and said second acoustic cell. 
     
     
       6. Apparatus as recited in  claim 3 , wherein said pump means includes a multi-stage piston pump.